1. Field of the Invention
The present invention relates in general to a reactant gas ejector head for use in chemical vapor deposition apparatus for forming thin films, and relates in particular to an ejector head suitable for use in forming high dielectric or ferroelectric films such as barium/strontium titanates.
2. Description of the Related Art
In recent years, there has been a remarkable progress in circuit density of integrated circuit devices produced by the semiconductor industry, and intense development activities are underway in anticipation of gig-bit order DRAMs replacing the prevailing meager-bit order DRAMs of today. Dielectric thin film materials used to make high capacitance devices necessary for producing DRAMs have, in the past, included silicon oxide or silicon nitride films of dielectric constant less than 10, tantalum pentaoxide (Ta.sub.2 O.sub.5) films of dielectric constant about 20; however, newer materials such as barium titanate (BaTiO.sub.3) or strontium titanate (SrTiO.sub.3) or mixtures of these compounds appear to be more promising.
Vapor deposition processes of such metal oxide thin films require one or a plurality of organo-metallic gaseous substances, termed a material gas, and an oxidizing gas, which must be mixed together uniformly, heated to a specific temperature and directed onto a substrate as a reactant gas flow. Some gas ejector heads have been proposed for such a purpose.
However, the conventional reactant gas ejector heads are unable to satisfy the rigorous requirements of organo-metallic chemical vapor deposition processes. In general, such gaseous mixtures of organo-metallic material gas and oxidizing gas, termed a process gas, can exist in a stable thermodynamic state only in a narrow range of temperatures, so that if the temperature distribution is non-uniform along its pathway to the substrate, component gases in the process gas can easily condense or decompose. For example, as the flow passage of the gaseous mixture becomes longer, the mixture of material gas and oxidizing gas can easily be affected by temperature variations to become thermodynamically unstable, permitting some undesirable premature reactions to take place before it reaches the substrate so as to produce unintentional depositions. Such depositions may plug up gas ejection nozzles or may flow downstream to contaminate the deposition film on the substrate.
If, on the other hand, the material gas and the oxidizing gas are mixed after they have passed through the ejection nozzle, although plugging of the ejection nozzles may be avoided, it becomes extremely difficult to uniformly mix the gases in the short distance available to the substrate. Attempts to produce uniform mixing by making the ejection nozzles finer or increasing the distance between the ejector head and the substrate are not practical solutions, because such approaches will only make the apparatus more complicated and undesirably large.